Saturday, December 15, 2012

Inversion of Control and Dependency Injection is a core design pattern of
Spring framework. IOC and DI design pattern is also a popular design pattern interview question in Java.
As name suggest Inversion of control pattern Inverts responsibility of managing life cycle of object e.g. creating object, setting there dependency etc from
application to framework, which makes writing Java application even more easy.
Programmer often confused between IOC and DI, well both words used interchangeably in Java world but Inversion of Control is more general
concept and Dependency Injection is a concrete design pattern. Spring
framework provides two implementation of IOC container in form of Application Context and BeanFactorywhich manages life-cycle of bean used by Java application. As you may know
necessity is mother of invention, it benefit to first understand problem solved
by IOC and Dependency Injection design pattern. This makes your understanding more
clear and concrete. We have touched basics of Dependency Injection and Inversion
of control in our article 10 OOPS and SOLID design principles for
Java programmer and this Java article tries to explain it by taking
a real life example of Service based architecture popular in enterprise Java
development. In this Spring or design pattern tutorial we will first see
normal implementation of AutditService class, a class in this example which
provides auditing in enterprise Java application and than use of dependency Injection.
This will allow us to find out problems
and how they are solved by Dependency
injection design pattern. . Also there are multiple way to inject
dependency in spring e.g. Setter Injection or Constructor Injection, which uses
setter method and constructor for injecting dependency, see Setter injection vs Constructor injectionto find out when to use them.

Inversion of Control and Dependency
Injection design pattern

Any way let’s back to core concept
of Inversion of Control and dependency Injection design pattern. Look at below
implementation of an AuditService whose job is to store every audit
messages into database. This is one of the simplest kind of auditing Service
required in Enterprise Java application.

In first glance this implementation looks perfect but there are three
major problem with this implementation:

1) Every AuditServiceImpl has its own copy of AuditDAO which is
an expensive object as it wraps adatabase connection with in.
It make no sense to create separate instances of AuditDAO, if you
can share one between multiple AuditService.

2) AuditServiceImpl is closely coupled with AuditDAO as its creating instance of AuditDAO using new() operator.
If you change the constructor of AuditDAO this code
will be broken. Because of this coupling its difficult to replace AuditDAO with
better implementation.

3) There is no easy way to test audit() method which is dependent on auditDAO. Since you
can not mock AuditDAO you have to rely on actual implementation and if AuditDAO is an environmental dependent
object which it is as it connect to different database on different
environment, your Junit test case may pass in
some environment and may fail in other environment.

What is Dependency Injection concept:

Dependency Injection is a design pattern on which dependency of object
(in this case AuditDAO is a dependency for AuditServiceImpl Object) is
injected by framework rather than created by Object itself. Dependency
Injection reduces coupling between multiple object as its dynamically injected
by framework. One of the implementation of DI is Inversion of Control (IOC) on
which framework like Spring controls object’s dependency. There are mainly two
types of Dependency Injection: Constructor Injection and Setter
Injection.

In Constructor Injection, dependency of Object is injected using constructor, while in Setter
Injection, Dependency is provided by setter method. Both has there
pros and cons. Constructor DI allows object to be created in complete state and
follows principle of fully functional object while Setter DI allows object to
be created without its dependency. which may result in incomplete object
if dependency is not available. This answers one of the famous spring interview question"when
do you use Setter injection and Constructor Injection in Spring".Another benefit of Setter Dependency
Injection is readability, since Spring is configured with xml configuration
file and setter injection is provided with bean property which is much
easier to read and understand than constructor injection which doesn't state
the property.

AuditServiceImpl
written using Dependency Injection

Now we will see How Dependency Injection solves all three problems we
have listed with above implementation of AuditService. here is a
new implementation of AuditService with setter dependency injection.

1. Since AuditDAO is injected here its possible to
share single AuditDAO (an expensive object) between
multiple AuditService.

2. Since AuditServiceImpl is not creating instance of AuditDAO its no
more coupled with AuditDAO and work with any implementation
of AuditDAO, thanks to another famous object oriented design principle “program for interface than
implementation".

3. Because AuditDAO is injected by DI at runtime its
easy to test audit() method by providing a mock AuditDAO class.
This not only makes testing easier but also independent of environmental
changes as you are not using actual implementation of AuditService.

This was the exact way I learn Dependency Injection and Inversion
Of Control design principles. It always help first to understand problem
and than solution to related each other. From above learning we can easily
derive advantages or benefits of
Dependency Injection in Java application:

1) Reduce
coupling

both constructor and setter dependency injection reduce coupling. like in
above example coupling between AuditService and AuditDAO is reduced
by using Dependency Injection.

This is another advantage which comes as side benefit of reduced
coupling, because of DI you can replace non performance implementation with
better one later.

That’s all on What is Inversion of control and Dependency Injection
design pattern. We have tried to learn this pattern with a real life
example and compares a class which is written using principle of IOC and DI and
without that. IOC and DI easily bring quality in coding. We have seen clear
benefits in terms of reduce coupling, improved testability and Flexibility to change implementation. It’s
always good to write code which follows principle of Inversion of Control and
dependency Injection and Spring framework by default ensures that.

4 comments
:

Ian
said...

I was confused between Inversion of Control and Dependency Injection and looking for exaxt deference between Inversion of Control and Dependency Injection. Your article explains it right for once and all. a Big Thanks

As the name implies Inversion of control means now we have inverted the control of creating the object from our own using new operator to container or framework. Now it’s the responsibility of container to create object as required. We maintain one xml file where we configure our components, services, all the classes and their property. We just need to mention which service is needed by which component and container will create the object for us. This concept is known as dependency injection because all object dependency (resources) is injected into it by framework.